1. Field of the Invention
The present invention relates to a fixing device for an image recording apparatus and, more particularly, to a fixing device incorporated in an image recording apparatus, such as a printer or a copying machine, for fixing an image formed on a sheet by heating a toner constituting the image while rotating an endless belt.
2. Description of the Related Art
Recently, a fixing device, which can reduce a warm-up time, has been developed. The fixing device is described by Kimura et al. in a publication entitled "New fixing system `SURF` can reduce the warm-up time", Japan Hardcopy '90 Documents of The Annual Conference of Japan Hardcopy for the Society of Electrophotography of Japan, pp. 53 to 56, Jun. 20-22, 1990. The fixing device of Kimura et al. includes a heater unit having a heating register line, and a thin endless belt. The thin endless belt is given tension by a drive roller, a tension roller and the heater unit, and it is rotated by the drive roller to carry a paper which is charged with toner constituting an image. While the paper is carried by the thin endless belt, it is pressed to the heater unit via the thin endless belt by a pressure roller so that the toner can be fixed on the paper by heat. However, because of the size or position fault of the two rollers and the imbalance of the tension to the tension roller and the heater unit, the thin endless belt skews.
To prevent the skew, an adjustment mechanism, for adjusting the position of the thin endless belt, is mounted in the fixing device. The conventional adjustment mechanism has a photosensor located at the side of the thin endless belt for detecting the skew, a solenoid operative in response to the photosensor, and a mechanical device connected to the solenoid for lifting or lowering one end of the tension roller. When the photosensor detects the skew of the thin endless belt, the solenoid activates the mechanical device to lift or lower the one end of the tension roller. The lifting or lowering directions are perpendicular to the tension direction of the tension roller. This mechanism is described in detail later.
However, the lifted or lowered tension roller imparts a strong tension to the thin endless belt because the belt gets twisted by lifting or lowering the one end of the tension roller, and this causes the belt to wear out prematurely. Moreover, since the photosensor and the solenoid are expensive, the above fixing device is not economical.